1. Field of the Invention
This invention relates to a developing device for developing an electrostatic image on an image carrying member which is used in an image forming apparatus, such as an electrophotographic apparatus, an electrostatic recording apparatus or the like.
2. Description of the Related Art
FIG. 6 illustrates a conventional developing device which uses a one-component magnetic developer (developing powder). This developing device includes a developing receptacle 3 containing a magnetic toner T, serving as a one-component magnetic developer. A developing sleeve 1a, serving as developer supporting means, is provided within the developing receptacle 3 at an opening facing a photosensitive drum 100, serving as an image bearing member, so as to be rotatable in the direction of the arrow "a". The developing sleeve 1a comprises a nonmagnetic member and incorporates a nonrotating magnet 1b, serving as magnetic-field generation means. A developer conveying member 4 is provided at a rear portion of the developing receptacle 3, and conveys the toner T toward the developing sleeve 1b. A magnetic blade 2 is provided above the developing sleeve 1a at a portion of the opening of the developing receptable 3. One of magnetic poles N of the magnet 1b within the developing sleeve 1a faces the magnetic blade 2 to provide a developer regulating portion. The magnetic blade 2 is disposed so as to maintain a constant predetermined gap W with the developing sleeve 1a. In general, the gap W is set within the range of 100 .mu.m-1 mm.
In the above-described developing device, the magnetic toner T contained in the developing receptacle 3 is carried on the developing sleeve 1a by the force of the magnetic field generated by the magnet 1b, and is conveyed toward a developing region facing the photosensitive drum 100 by the rotation of the developing sleeve 1a. The magnetic toner T is regulated at the regulating portion by the magnetic blade 2, and is coated as a thin layer on the developing sleeve 1a. As shown in FIG. 7, the thickness of this thin toner layer is determined by the position of a broken line L which passes between the developing sleeve 1a and the magnetic blade 2 so as to be parallel with the surface of the developing sleeve 1a.
According to the investigations of the inventor of the present invention, the provision of electric charges, the conveying mechanism, and the behavior of the magnetic toner T when it passes between the developing sleeve 1a and the magnetic blade 2 have turned out to relate as will now be discussed.
As shown in FIG. 8, two planes perpendicular to a line obtained by connecting the magnetic blade 2 to the developing sleeve 1a are considered. The plane closer to the magnetic blade 2 is designated by .alpha.1, and the plane closer to the developing sleeve 1a is designated by .alpha.2. In general, since the width of the magnetic blade 2 (the length in the circumferential direction of the developing sleeve 1a) is narrower than the width of the magnetic pole N of the magnet 1b, the magnetic flux density of the magnetic field from the magnetic pole N of the magnet 1b at the plane .alpha.1 is greater than that at the plane .alpha.2. Hence, the magnetic toner T carried on the developing sleeve 1a is under the influence of a magnetic force converging toward the magnetic blade 2 between the developing sleeve 1a and the magnetic blade 2, as indicated by the arrows "h" in FIG. 8.
As a result, as indicated by B in FIG. 7, the magnetic toner T forms "ears" issuing from the magnetic blade 2 toward the developing sleeve 1a between the magnetic blade 2 and the developing sleeve 1a. Toner particles t1 at the distal ends of the ears contact the developing sleeve 1a, and triboelectric charges are supplied to the toner particles t1 at the distal ends of the ears of the magnetic toner T.
The toner particles t1 at the distal ends of the ears of the toner T, to which the triboelectric charges are supplied to develop a latent image, adhere to the developing sleeve 1a due to an electrostatic mirror force. The toner particles t1 also are provided with a conveying force in the direction of the rotation of the developing sleeve 1a due to a frictional force with the developing sleeve 1a. At that time, since a certain amount of cohesive force exists between respective toner particles, a conveying force as a result of the cohesive forces is also generated in toner particles t2 in the second layer contacting the toner particles t1 at the distal ends of the ears. Similarly, a conveying force as a result of the cohesive force is generated in toner particles t3 in the third layer immediately above the second layer t2.
However, the above-described magnetic force in the direction of the magnetic blade 2 is exerted on toner particles between the developing sleeve 1a and the magnetic blade 2. Accordingly, the conveying force exerted on the toner particles exceeds the magnetic force at a certain position. If it is assumed that this position corresponds to the above-described broken line L, the ears of the toner particles are disconnected at the broken line L, and the toner particles at the side of the developing sleeve 1a are conveyed in the direction of rotation thereof.
On the other hand, as indicated by A shown in FIG. 7, toner particles having insufficient electric charges are present at the side of the magnetic blade 2. If a toner pool formed by these remaining toner particles grows, the magnetic force cannot hold the toner particles on the magnetic blade 2, and a group of toner particles having insufficient electric charges breaks away from the toner pool and is conveyed in the direction of the rotation of the developing sleeve 1a.
As is apparent from the foregoing description, sufficient electric charges can be supplied only to the toner particles t1 in the first layer on the developing sleeve 1a, and some toner particles conveyed by the developing sleeve 1a are not supplied with the necessary electric charge. As a result, conventionally, in some cases, development becomes unstable due to unstable charging of toner particles, and therefore high quality images cannot be stably obtained.
In order to solve the above-described problems, the assignee of the present application has proposed, in U.S. Pat. No. 5,517,286, application Ser. Nos. 08/250,682 and 08/348,222, devices for regulating the thickness of the layer of a developer using a layer-thickness regulating rotating member rotating in a direction opposite to a developing sleeve.
FIG. 9 illustrates one such device. This developing device includes a developing receptacle 3 for containing a magnetic toner T, serving as an insulating one-component magnetic developer. A developing sleeve 1a for receiving the magnetic toner T is provided so as to be rotatable in the direction of the arrow "b" within the receptacle 3 at an opening facing an electrophotographic photosensitive drum 100 rotating in the direction of the arrow "a". The developing sleeve 1a is made of a nonmagnetic material, such as aluminum or the like, and incorporates a nonrotating magnetic roller 1b. Two adjacent developer conveying members 4a and 4b for conveying the magnetic toner T within the receptacle 3 to the developing sleeve 1a by rotating in the directions of the arrows "c" and "d" are provided at a rear portion of the developing receptable 3.
According to this method, a thickness regulating means 6A, comprising a regulating sleeve 6a made of a nonmagnetic material, such as aluminum or the like, and a nonrotating magnet roller 6b incorporated therein, is provided close to the developing sleeve 1a at an upstream side from a developing region provided by the photosensitive drum 100 and the developing sleeve 1a facing each other in the direction of the rotation of the developing sleeve 1a. The regulating sleeve 6a rotates in the direction of the arrow "e", i.e., in the same direction as the developing sleeve 1a. That is, the developing sleeve 1a and the regulating sleeve 6a move with each other at a portion where the distance between the two sleeves is smallest.
A nonmagnetic elastic scraper 7, made of a synthetic resin or the like, for removing toner particles adhering to the regulating sleeve 6a contacts the surface thereof.
In FIG. 9, the stationary magnet roller (a permanent magnet) 1b within the developing sleeve 1a has six magnetic poles, S1, S2, S3, N1, N2 and N3, where S indicates a south pole, and N indicates a north pole.
The magnetic pole N2 is a developing magnetic pole for generating a magnetic field in the developing region, and is disposed where the distance between the developing sleeve 1a and the photosensitive drum 100 is smallest. The magnetic pole N1 has a function of regulating the toner layer in cooperation with the regulating means 6A, as will be described later. The other magnetic poles S1, S2, S3 and N3 have a function of magnetically attracting toner particles onto the magnetic sleeve 1a to assist conveyance of the toner particles caused by the rotation of the developing sleeve 1a.
On the other hand, in FIG. 9, the stationary magnet roller (a permanent magnet) 6b within the regulating sleeve 6a has two magnetic poles S4 and N4, which are positioned so that the magnetic pole N1 and the magnetic pole S4 having an opposite polarity magnetically attract each other. Accordingly, magnetic lines of force are continuous between the two magnetic poles, and a strong magnetic field is generated in a gap W between the developing sleeve 1a and the regulating sleeve 6a, i.e., between the magnetic poles N1 and S4.
The magnetic field generated between the magnetic poles N1 and S4 prevents the magnetic toner from flowing through the gap W between the developing sleeve 1a and the regulating sleeve 6a, i.e., the regulating portion toward the developing region.
Since the regulating sleeve 6a moves in a direction opposite to the moving direction of the developing sleeve 1a where they face each other, a frictional force is exerted on the toner contacting the regulating sleeve 6a by the function of the above-described magnetic field to provide a conveying force in the direction of rotation of the regulating sleeve 6a, i.e., a conveying force in a direction opposite to the conveying direction by the developing sleeve 1a. This conveying force is also transmitted to toner particles remote from the regulating sleeve 6a due to the frictional force and the cohesive force exerted between respective toner particles. As a result, the conveying force from the regulating means 6A in the direction of rotation of the regulating sleeve 6a, i.e., in a direction toward the rear side of the receptacle 3, is exerted on the toner at the regulating portion.
As described above, electric charges generated by friction with the developing sleeve 1a are supplied to the magnetic toner in the first layer contacting the developing sleeve 1a. The toner is attracted onto the developing sleeve 1a by the mirror force produced by the electric charges, and a conveying force in the direction of rotation of the developing sleeve 1a is exerted on the toner by the frictional force with the developing sleeve 1a.
Since such a developing device can supply the developing portion with only sufficiently charged toner particles, the quality of the developed image is improved.
It is necessary to seal the spaces between the receptacle 3 and the developing sleeve 1a and between the receptacle 3 and the regulating sleeve 6a in order to prevent toner particles adhering to the surface of the sleeves from flowing in the axial directions of the sleeves. For that purpose, sealing members (made of a wool, felt, or the like) may be positioned in pressure contact with the surface of each of the sleeves. As a result, the driving torque for rotating the regulating sleeve 6a and the developing sleeve 1a increases.
In the device shown in FIG. 9, a certain amount of driving torque for rotating the regulating sleeve 6a is required. In addition, since the elastic scraper 7, serving as a cleaning member, contacts the surface of the regulating sleeve 6a in order to remove toner particles adhering thereto, a considerable additional amount of driving torque for rotating the regulating sleeve 6a is required.
As a result, the driving torque required for the developing device becomes considerably higher than when using a magnetic blade as shown in FIG. 6. A larger driving torque naturally requires a higher-performance drive motor, resulting in increases in cost and space.
Since a high driving torque tends to produce nonuniform rotation at an interlocking pitch between gears or the like, smooth rotation of the sleeve cannot be obtained, and the obtained image may be degraded.
In a driving system shown in FIG. 12, an increase in the torque of a regulating sleeve 6 results in an increase in a force F.sub..theta. generated between a gear 6g of the regulating sleeve 6 and a gear 25g meshing with the gear 6g. As shown in FIG. 13, an increase in the force F.sub..theta. generates a reaction force F'.sub..theta. making the receptacle 3 function as a fulcrum. As a result, a central portion of the regulating sleeve 6 is deflected in the direction of an arrow Z shown in FIG. 12. If the central portion of the regulating sleeve 6a is deflected, the gap W (FIG. 9) between the regulating sleeve 6a and the developing sleeve 1a becomes nonuniform in the axial directions of the sleeves. That is, the gap W has a smaller value at the central portion in the axial directions of the sleeves. Since, as shown in FIG. 14, the gap W greatly influences the thickness of the toner layer, the thickness of coated toner particles at the central portion becomes, in some cases, smaller than that at end portions of the sleeves.
If the contact pressure of the sealing members is reduced in order to reduce such an increase in the driving torque, the effectiveness of sealing is reduced.
In addition, since both ends of the developing sleeve and the regulating sleeve must be sealed, the sealing members must have a complicated configuration.